Generally, a transmission control unit (TCU) controls all operations of an automatic transmission's shifting by controlling the operation of valves selectively communicating lines to control the flow of hydraulic pressure.
Solenoid valves are connected to the valves to control the opening and closing of the same such that hydraulic pressure flow and levels can be precisely controlled.
Because viscosity of oil used in the transmission undergoes changes according to temperature fluctuations, it is possible for the pressure of the oil to be different from that intended by control of the solenoid valves.
Accordingly, transmissions use oil temperature sensors to enable the TCU to adjust electrical current levels applied to the solenoid valves according to the detected temperature. This allows hydraulic pressure levels to be precisely controlled by compensating for temperature changes such that precise shift timing and a high level of shift quality can be maintained.
So, for example, if oil viscosity is increased as a result of a reduction in oil temperature, the TCU controls the amount of electrical current applied to the solenoid valves such that hydraulic pressure levels can be adjusted to compensate for the reduction in the flow of oil caused by an increase in viscosity.
In the above prior art system for detecting oil temperature in the automatic transmission, analog signals output from the oil temperature sensor, which is provided in the automatic transmission (the automatic transmission being grounded to the vehicle body), are converted to digital signals by an analog/digital (A/D) converter. Then, a method is used in which a microprocessor recognizes the digital signals.
However, in the prior art oil temperature detecting system it is possible for a difference in electric potential to occur in the ground of the transmission and vehicle body, especially in older vehicles. To remedy this, a cable is used in the prior art to connect the vehicle body to the automatic transmission. But short-circuiting and disconnection problems often occur in the cable such that the ground potential becomes different between the transmission and vehicle body. In either case, errors in the signal output from the oil temperature sensor result.
Accordingly, these errors in the oil temperature signals cause the TCU to incorrectly compensate for changes in oil viscosity such that shift shock is generated.